The present invention relates to a railway track circuit, formed by the two rails of a railway track portion and comprising a transmitting member connected to the downstream end of the circuit and a receiving member connected to the upstream end.
It is known that the safety and the regularity of trains running on railway tracks depend, among other conditions, on the distance separating two successive trains on the same track, taking into account the admissible speed with respect to the braking characteristics of the trains and the profile of the line.
The information required by the driver of the train for initiating actions for ensuring such safety and such regularity may be transmitted at fixed points of the route by lateral signals spaced out along the tracks. They may also, as a substitution for or as a reinforcement of the lateral signalling and when it is a question of automatic driving or of controlled manual driving, be transmitted directly at all points of the track to the locomotive.
Generally, at the present time, these are safety devices called "track circuits", which enable the information to be elaborated and transmitted required for the safety and the regularity of the traffic, not only in lateral signal systems but also in a number of systems using processes for transmitting information from the track to the locomotive.
In a way known per se, the track is divided into a succession of sections, each section being equipped with a track circuit. In the most general form, a track circuit is formed by a transmitting member and a receiving member, each situated at one end of the track circuit, and connected to the rails, so that a shunt axle between the transmitting point and the receiving point of the track causes the de-energization of a relay associated with the receiver. In the case of a track circuit associated with lateral signals, the relative position of the transmitter and of the receiver of the track circuit with respect to the entry and the exit of the section is immaterial, since only the presence or the absence of a shunt axle in the section counts. The same cannot be said in the case where the track circuit is used in a system with transmission of information from the track to the train. In such a system, the train receives the information by picking up the electromagnetic field radiated by the rails, which field exists because of the flow of signalling current in each of the lines of rails. The receiving member situated on board the train must then, on principle, be permanently located between the transmitting member and the first shunt axle of the train. It follows then obviously that in this case the transmitting member must always be connected to the downstream end of the track circuit, whereas the receiving member is connected to the upstream end.
In rail networks where the density of the traffic is one of the dominant elements, such as urban networks, the spacing signalling must be designed so that the distance separating two successive trains is minimized and that the time spent by trains in front of a closed signal is reduced as much as possible. It is therefore advantageous to be able to open the signal by activating the freeing, by the train occupying it, of a section situated downstream, while keeping between the signal to be opened and a critical point of the section being freed a free length of track corresponding to the maximum braking distance under the most unfavourable conditions. It is necessary, to achieve such anticipation, to know with all the required safety the position of the whole of the train with respect to both ends of the section which it occupies and/or with respect to the possible critical points.
Now, in the known systems of the prior art, the requirement of locating simultaneously the first shunt axle of the train (head of the train) and the last shunt axle of the train (tail end of the train) so as to know the relative position of the whole of the train with respect to both ends of the section and/or to a particular point leads to incompatibility between track circuit and transmission of information from the track to the locomotive.